Method to control nested to isolated line printing

ABSTRACT

A method and structure for a photomask that includes a substrate having a first transmittance, a first pattern to be transferred to a photosensitive layer (the first pattern having a second transmittance lower than the first transmittance) and a second pattern having a third transmittance greater than the second transmittance and less than the first transmittance. The second pattern is adjacent at least a portion of the first pattern, and the substrate and the second pattern transmit light substantially in phase.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to microlithographic printedpatterns and structures and more particularly to improvements whichdecrease size differences between isolated and nested structures.

2. Description of the Related Art

As the minimum feature size in semiconductor integrated circuittechnology is pushed near or below the wavelength of the light used inmicrolithographic projection printing, diffraction effects introducesignificant differences between the patterns used on microlithographicreticles and the resulting printed structures on a semiconductor wafer.Similarly, the smaller the circuit elements become, the more difficultit becomes to create the desired pattern shapes on the wafer due tofactors such as localized etch variations, mask distortions, lensdistortions, topography variations, and non-uniform materialcomposition.

These physical factors introduce deviations in the dimension of printedisolated structures versus printed nested structures, with the degree ofdeviation being highly dependent on the degree of proximity of nearbyshapes. In order to maximize circuit performance and speed, it has beenadvantageous to make the device structure dimensions as identical aspossible (e.g., to have isolated gates and nested gates print asidentically as possible). These effects become increasingly important asthe physical dimensions of the circuit elements decrease. However, it isdifficult to make isolated and nested structures print as identically asdesired resulting in an undesirable condition known as across chip linewidth variation (ACLV).

ACLV is a major problem in semiconductor device fabrication. Image sizevariations can affect transistor speed matching and resistivity andconductance matching from one portion of the chip to another.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide astructure and method for a photomask comprising a substrate having afirst transmittance, a first pattern to be transferred to aphotosensitive layer (the first pattern having a second transmittancelower than the first transmittance) and a second pattern having a thirdtransmittance greater than the second transmittance and less than thefirst transmittance. The second pattern is adjacent to at least aportion of the first pattern, and the substrate and the second patterntransmit light substantially in phase.

The first pattern includes nested structures and isolated structures andthe second pattern is adjacent an outer edge of nested structures andthe isolated structures. The nested structures are spaced more closelythan the isolated structures. The second pattern may be positionedbetween the nested structures.

The first pattern includes a pair of outer lines and at least one innerline and the second pattern is adjacent an edge of at least one of theouter lines or an edge of the inner line.

The invention also includes a method of preparing a photomask thatincludes supplying a substrate having a first transmittance, forming afirst pattern to be transferred to a photosensitive layer (the firstpattern having a second transmittance lower than the firsttransmittance) and forming a second pattern having a third transmittancegreater than the second transmittance and less than the firsttransmittance. The second pattern is adjacent at least a portion of thefirst pattern, and the substrate and the second pattern transmit lightsubstantially in phase.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 shows mask nested and isolated lines on the left with thecorresponding wafer level aerial images on the right;

FIG. 2 shows corrected mask nested and isolated lines on the left withthe corresponding wafer level aerial images on the right where nestedlines print the same as isolated lines;

FIG. 3 shows mask nested and isolated lines on the left with thecorresponding wafer level aerial images on the right where nested linesprint small relative to isolated lines;

FIG. 4 shows corrected mask nested and isolated lines on the left withthe corresponding wafer level aerial images on the right where nestedlines print the same as isolated lines; and

FIG. 5 is a flow diagram of an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

As mentioned above, ACLV is a major problem in semiconductor devicefabrication. One significant cause of printed pattern size variation isthe diffraction component of imaging, which results in structures on areticle being imaged differently depending upon what other reticlestructures are present in the local neighborhood. A common form of thisis the printed image variation that occurs between nested and isolatedstructures.

All the examples herein which describe the invention use a positive mask(the indicated mask shapes are made with an opaque material such aschrome) to expose a positive resist, thereby resulting in resiststructures that appear close in shape and position to the opaque maskpatterns. However, as would be known by one ordinary skilled in the artgiven this disclosure, the invention is equally applicable to all typesof masking processes, such as negative photoresist processes.

Further, while the structures are classified herein as “nested” (e.g.,being adjacent other structures) and “isolated” (e.g., standing alone),these terms are not intended to limit the application of the invention.Since items in printed patterns are always somewhat “adjacent” otheritems, the same structure could be nested with respect to some structureand at the same time be isolated with respect to other structures. Thus,the relative nature of the terms “nested” and “isolated” does not limitthe application of the invention. To the contrary, such terms are merelyused to illustrate the operation of the invention with respect to somespecific examples (such as those shown in FIGS. 1-4).

The invention places “gray” regions 10, 11 in specific locations of themask to correct the nested and isolated printing mismatch. The inventionproduces significant improvement in the printability of nested toisolated patterns by adding gray regions 10, 11. “Gray,” as used herein,has a magnitude of transmittance between 0% and 100% and preferably atransmittance of about 85%.

In the examples used herein, the phase of these gray regions 10, 11 isequal to the phase of the clear regions of the mask. However, theinvention is equally applicable to phase shift masks. As would be knownby one ordinarily skilled in the art given this disclosure, altering thephase of the “gray” regions 10, 11 can also be used as a “knob” toimprove performance. In the following examples the gray regions are usedwith phase zero (as compared with the clear regions) and with atransmittance of about 85%.

Further, while the following examples are discussed with respect to asize of 0.25 microns, as would be apparent to those ordinarily skilledin the art, the invention is equally applicable to any dimensions ofinterest, for example, 0.175 microns. In FIGS. 1 through 4, the leftside of each figure represents the mask design while the right siderepresents the printed structures.

The nested lines, as shown in the left side of FIG. 3 (E1 through E4),usually print smaller (F1 through F4) than the isolated line (E5 versusthe printed line of F5). Thus, ACLV is apparent as F1 through F4 aresmaller in width than F5.

Although this behavior is what is typically observed for line/spacedimensions of current interest (i.e., nested lines printing smaller thanisolated lines for a positive mask with positive resist), this behavioris not always seen if, for example, the period of the line/space patternis sufficiently large. Indeed, FIG. 1 represents the opposite case,where nested mask images A1 through A4 print larger (B1 through B4) thanisolated mask image A5 prints (B5).

This disclosure illustrates the invention using gray regions in twocases. However, as would be known by one ordinarily skilled in the art,the invention is equally applicable to other similar printing systems.The first case, as represented by FIGS. 1 and 2, involves the situationwhere the nested lines print larger than the isolated line, while FIGS.3 and 4 involve the situation where the nested lines print smaller thanthe isolated line, which is the problem observed more often in currenttechnologies.

Regarding case 1, where the nested images print larger than the isolatedimage, there are also size differences between the printed nested imagesthemselves. More specifically, the inner mask images (A2 and A3) printlarger (B2 and B3) than the outer mask images (A1 and A4) will print (B1and B4). The reason for this result is that the inner lines are more“nested” than the outer lines, so the outer lines will mimic thebehavior of the isolated line (A5 and B5) more than the inner lines. Thesame result occurs in case 2 (FIGS. 3 and 4), where the nested linesprint smaller than the isolated lines. More specifically in FIGS. 3 and4, the outer lines (F1 and F4) are closer in character to an isolatedline (F5), therefore, they print closer in dimensions (although stillsmaller than a fully isolated line) than do the inner lines (F2 and F3).

For case 1, placing gray regions 10 outside the isolated mask line C5and outside the outer nested mask lines C1 and C4, as shown in FIG. 2,results in printed structures shown by D1 through D4, which have acloser dimension to D5. Thus, for example, in FIG. 1, the nested maskpattern structures of A1 through A4 and isolated mask pattern structureA5 may all be 0.25 microns wide. The exposure is set to print images D2and D3 0.25 microns wide. If the remaining images D1, D4 and D5 are notalso 0.25 microns wide, then mask compensation is applied to all images.Ignoring effects which occur at the line ends, resist nested images B1and B4 print 0.275 microns wide and resist nested images B2 and B3 print0.30 microns wide when the exposure has been selected to print isolatedresist image B5 0.25 microns wide. This ACLV is resolved with theinvention as shown in FIG. 2.

In FIG. 2 nested mask pattern structures C1 through C4 and isolated maskimage C5 are all (for example) 0.25 micron wide because of the grayareas 10. Ignoring effects which occur at the line ends, all resistimages B1 through B5 can be made to print 0.25 microns wide by applyinggray zones 10 to the outer edges of outer nested mask images C1 and C4.The exposure is set to print images D2 and D3 0.25 microns wide.

If the remaining images D1, D4 and D5 are not also 0.25 microns wide,then mask compensation is applied to all images. Mask compensation isthe adjustment of the size of all shapes so as to achieve the desirednominal line width on the wafer once differences in printed line widthsdue to proximity have been corrected.

Continuing with a similar example, in FIG. 3 nested mask images E1through E4 and isolated mask image E5 are all (for example) 0.25 micronwide. Ignoring effects which occur at the line ends, resist nestedimages F1 and F4 print (for example) 0.225 microns wide and resistnested images F2 and F3 print 0.20 microns wide when the exposure hasbeen selected to print isolated resist image F5 0.25 microns wide. Thisdifference is corrected with the invention as shown in FIG. 4.

More specifically, in FIG. 4, nested mask images G1 through G4 andisolated mask image G5 are all (for example) 0.25 micron wide. Ignoringeffects which occur at the line ends, all resist images H1 through H5can be made to print 0.25 microns wide by applying gray zones 11 betweenmask images G1 and G2, G2 and G3, and G3 and G4. The exposure is set toprint the isolated image H5 0.25 micron wide.

The reasons why adding the gray regions 10, 11 helps make the nestedstructures print more like isolated ones can perhaps best be understoodby examining the spaces, rather than the lines. In FIG. 3, the nestedlines F1-F4 print too narrow as compared with the isolated line F5, or,the nested spaces print too wide to enable the nested lines F1-F4 toprint as the isolated line F5. To make the spaces print narrower,thereby making the lines F1-F4 print wider, the invention inserts thegray regions 11, thereby making the resist regions that are eventuallydissolved away smaller in width, thereby achieving the desired result.The gray regions 11 affect the “inner” spaces more than the outer ones.This compensates G2 and G3 more than G1 and G4 which reduces the widthamong differences the nested F1-F4 lines, as discussed above.

For the other case of FIG. 1, the nested lines A1-A4 print as widerlines (B1-B4) than the isolated line B5 because the spaces print toonarrow. To make them print wider, more light needs to be delivered tothe nested line/space structure, as compare with the light delivered tothe isolated line structure. This can be accomplished by adding the grayregions 10 around the isolated line, as indicated by C5 in FIG. 2, whichreduces the light delivered to the isolated line. To make the isolatedline D5 print the desired width, the exposure time needs to beincreased, which allows more light to enter the region of the nestedline structures D1-D4. This increases the widths of the spaces betweenthe lines and decreases the original width of the lines (D1 through D4are smaller in width than B1 through B4). Since, as discussed above, theouter lines B1 and B4 are not as wide as B2 and B3, the outer lines B1and B4 do not need to be adjusted as much, and in order to make theireffective proximity correction more like the isolated line, then grayregions can be placed only outside mask images C1 and C4 to make theprinted structure (D1 and D4) close in width to the isolated and to theinner nested line widths.

FIG. 5 illustrates a first embodiment of the invention in flowchartform. The process shown in FIG. 1 first supplies a substrate as shown initem 50. In item 51 a first pattern C1-C5, G1-G5 is formed and then asecond pattern 10, 11 is formed as shown in item 52. Finally, thephotoresist is exposed using the photomask as shown in item 53.

As discussed above, the invention corrects the difference between nestedand isolated printed structures using gray areas in the mask. Anotherway to reduce these differences is to simply bias the line dimensionsdifferently. However, depending on the dimensions involved, biasing theline dimensions differently is not always helpful, because it can hurtthe process window (e.g., depth of focus). To the contrary, theinvention improves the process window and allows additional flexibility.

Bias changes used to be limited to an integer multiple of the maskwriting grid size. However, the recent use of halftone biasing andsub-resolution jogs in place of line edges, enables sub-grid sizecorrections to line widths. This method significantly increases thevolume of the data to be written on the mask. The inventive gray areascan be used in combination with such bias change to fine tune theconsistency of the printed images.

The examples shown above all involved 4 “nested” lines. However, aswould be apparent to one ordinarily skilled in the art, the invention isnot limited to four straight structures but is equally applicable to anynumber of straight or differently shaped structures. This method can beused to control line width alone or in conjunction with othertechniques.

The increase in data volume is small and the size of the gray areas islarge enough to be printed by older mask writing tools. Thus, theproblems associated with printing and inspecting sub-resolution featuresare not encountered with this method. Further, different areas of a chipcan be adjusted independently by varying the sizes of the gray areas.Also, the use of multiple reduced transmittance films on the maskenables additional line width tailoring capability.

While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

What is claimed is:
 1. A photomask comprising: substrate having a firsttransmittance; a first pattern to be transferred to a photosensitivelayer, wherein said first pattern is opaque, and said first pattern hassecond transmittance, lower than said first transmittance; and secondpattern having a third transmittance greater than said secondtransmittance and less than said first transmittance, said secondpattern being applied to an outer edge of said first pattern, whereinsaid first pattern includes nested structures and isolated structuresand said second pattern is laterally adjacent only outer edges of saidnested structures and said isolated structures, wherein said nestedstructures are spaced more closely than said isolated structures, andwherein said substrate and said second pattern transmit light in-phase.2. The photomask in claim 1, wherein said first pattern includes a pairof outer lines and at least one inner line and said second pattern isadjacent an edge of at least one of said outer lines.
 3. The photomaskin claim 1, wherein said first pattern includes a pair of outer linesand at least one inner line and said second pattern is adjacent an edgeof said inner line.
 4. A method of preparing a photomask comprising:supplying a substrate having a first transmittance; forming a firstpattern to be transferred to a photosensitive layer, wherein said firstpattern is opaque, and said first pattern has a second transmittance,lower than said first transmittance; and forming a second pattern havinga third transmittance greater than said second transmittance and lessthan said first transmittance, said second pattern being applied to anouter edge of said first pattern, wherein said first pattern includesnested structures and isolated structures and said second pattern islaterally adjacent only outer edges of said nested structures and saidisolated structures, wherein said nested structures are spaced moreclosely than said isolated structures, and wherein said substrate andsaid second pattern transmit light-in-phase.
 5. The method in claim 4,wherein said first pattern includes a pair of outer lines and at leastone inner line and said second pattern is adjacent an edge of at leastone of said outer lines.
 6. The method in claim 4, wherein said firstpattern includes a pair of outer lines and at least one inner line andsecond pattern is adjacent an edge of said inner line.
 7. A photomaskcomprising: substrate having a first transmittance; a first pattern tobe transferred to a photosensitive layer, wherein said first patter isopaque, and said pattern has a second transmittance, lower than saidfirst transmittance; and a second pattern having a third transmittancegreater than said second transmittance and less than said firsttransmittance, said second pattern being applied to an outer edge ofsaid first pattern, wherein said first pattern includes a nestedstructures and isolated structures and said second pattern is positionedonly laterally between said nested structures, wherein said nestedstructures are spaced more closely than said isolated structures, andwherein said substrate and said second pattern transmit light in-phase.8. The photomask in claim 7, wherein said first pattern includes a pairof outer lines and at least one inner line and said second pattern isadjacent an edge of at least one of said outer lines.
 9. The photomaskin claim 7, wherein said first pattern includes a pair of outer linesand at least one inner line and said second pattern is adjacent an edgeof said inner line.
 10. A method of preparing a photomask comprising:supplying a substrate having a first transmittance; forming a firstpattern to be transferred to a photosensitive layer, wherein saidpattern is opaque, and said first pattern has a second transmittance,lower than said first transmittance; and forming a second pattern havinga third transmittance greater than said second transmittance and lessthan said first transmittance, said second pattern being applied to anouter edge of said first pattern, wherein said first pattern includesnested substrate and isolated structures and said second pattern ispositioned only laterally between said nested structures, wherein saidnested structures are spaced more closely than said isolated structures,and wherein said substrate and said second pattern transmit lightin-phase,
 11. The method in claim 10, wherein said first patternincludes a pair of outer lines and at least one inner line and saidsecond pattern is adjacent an edge of at least one of said outer lines.12. The method in claim 10, wherein said first pattern includes a pairof outer lines and at least one inner line and said second pattern isadjacent an edge of said inner line.